Tests hypotheses that breath tests using 13C-labelled methacetin and caffeine, coumarin disposition and urinary excretion of 6beta-hydroxycortisol can be validated as in vivo probes for the cytochrome P450 enzymes CYP1A2, CYP2A6, and CYP3 respectively, and that dietary factors (charcoal-broiled beef or dietary protein) will have demonstrable effects on the metabolism of these probes in normal subjects that may be comparable in magnitude to effects of cimetidine.